<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS选择器</title>
<style>
h1{
    color: blue;
    background-color: yellow;
}
.nameh2{
    color:red;
}
#classh3{
    color: red;
    background-color: blue;
}

</style>


</head>
<body>
    <h1>this is h1</h1>
    <h2 class="nameh2">this is h2</h2>
    <h3 id="classh3">this is h3</h3>
</body>
</html>